Wavelength division multiplexing (WDM) of optical beams is presently being used to increase the rate of transmission of information through an optical fiber. These multi-wavelength optical beams provide information by a plurality of signal components, also referred to as optical channels. Each channel is defined by a unique wavelength of light that are multiplexed together and transmitted through a communication link of an optical network. To date, the industry has provided WDM of only eight channels, however, the channels are broadly spaced and not well defined.
Currently, wavelength division multiplexed (WDM) intercity communications links, as shown in FIG. 1, require a number of amplifiers along the link length to compensate for fiber loss. As the intercity links can run approximately 600 meters, there is currently a need for an amplifier every 80 to 120 kilometers in these links. With ever expanding telecommunication networks, network configurations and number of WDM channels, these amplifiers will need to provide low-noise, uniform-gain to each channel over a wide range of operating conditions (such as during the addition and subtraction of channels).
Each of the optical gain amplifiers are composed of an optical fiber doped with a rare earth ion, such as erbium or praseodymium. The gain as a function of color, or gain spectrum, of these amplifiers are not uniform over the range of wavelengths of the channels. The nonuniform gain characteristic of the doped optical fiber is compounded each time the optical beam is amplified along the communication link. Another phenomenon of gain tilt occurs especially for dynamically changing and/or reconfigurable dense wavelength division multiplexed communication links, wherein a multi-wavelength optical beam has as many as 40 tightly spaced channels. The effect of gain tilt is a function of the input power and wavelength of each transmitted channel. When a channel is added or subtracted, and thus changes the input power and spectrum of the optical beam, a gain fluctuation occurs in dependence on the channels' wavelength to effectively "tilt" the gain of the amplifier.
FIG. 1 shows a graphical representation of a typical communication link 2 of an optical network. A plurality of light generators (LG) 3 provide respective component signals 12 of select wavelengths that are combined by a multiplexer 4 to produce the multi-wavelength optical beam 14. Before the component signals are multiplexed, a plurality of pre-emphasis devices (PE) 5 attenuate selectively each of the respective component signals 12. As mention hereinbefore, a plurality of amplifiers 20 amplify the optical beam 14 to compensate for fiber loss as the beam passes therethrough. At the receiver end of the communication link, the signal components 12 of the optical beam are then separated by a demultiplexer 6 and provided to a corresponding receiver (R) 7.
The prior art 2 does not provide any compensation other than pre-emphasis to overcome the nonuniform gain of the each amplifier 20. As shown in each of the plots 14, which are representative of the amplitude of the output power of each channel 12, the differential of the output power of each of the channels increase after each gain stage 20. The output power and signal-to-noise ratio of each channel of the communication link at 8, therefore, are not equal. The only compensation provided by the prior art is adjustment of the pre-emphasis devices 5 for amplifying each channel 12 a predetermined amount to ensure that the output power of each channel are of acceptable power and signal-to-noise ratio.
Accordingly, it is the principal object of this invention to provide an optical amplifier that equally amplifies each channel of a dense wavelength division multiplexed optical beam.
It is another object of this invention to provide an optical amplifier that compensates for the non-uniform gain spectrum of doped fiber gain amplifier.
It is a further object of this invention to provide an optical amplifier that compensates for dynamic fluctuations and variation resulting from the adding and subtracting of channels.
It is yet another function of this invention to provide a dynamic amplifier that requires no calibration which permits the amplifier to be interchangeable throughout the optical network.